1. Field of the Invention
This invention relates to a copper-base alloy that has a golden visual appearance. More particularly, the copper base alloy is useful, as either a monolithic material or as a cladding layer, for the stamping of planchets used to mint coins.
2. Description of Related Art
The Susan B. Anthony United States one dollar coin (SBA) is a three-layer clad with a copper alloy C110 core and copper alloy C713 cladding layers bonded to opposing sides of the core. Copper alloy C110 has a nominal composition, by weight, of 99.95% copper and 0.04% oxygen and is commonly known as electrolytic tough pitch (ETP) copper. Copper alloy C713 has a composition, by weight, of 23.5%-26.5% nickel and the balance copper. Copper alloy C713 is commonly known as a cupronickel.
In the SBA, the ETP copper core constitutes about 50% of the coin""s overall thickness and each of two cupronickel cladding layers constitutes about 25% of the overall thickness.
A significant problem with the SBA is that the coin has a silver/gray visual appearance that is similar to the color of the United States quarter. Since the SBA and the quarter, which has only 0.25 the face value of the SBA, are also similar in size and weight, it is not uncommon for the SBA to be mistaken for a quarter.
It is desirable for the United States to have a one dollar coin that is readily distinguished from the quarter. The United States Mint has suggested that a new one dollar coin have a golden visual appearance.
A number of gold-colored materials for coinage are known. U.S. Pat. No. 4,401,488 to Prinz, et al. discloses a copper base alloy containing, by weight, 4%-6% of nickel and 4%-6% of aluminum. xe2x80x9cCopper basexe2x80x9d is intended to convey that the alloy contains at least 50%, by weight, of the base material, copper.
U.S. Patent No. 4,330,599 to Winter, et al. discloses a copper base alloy containing, by weight, 2%-3.5% of aluminum and 1%-2.5% of silicon. A number of visually golden copper alloys are disclosed in U.S. Pat. No. 5,472,796 to Breedis, et al. The alloys disclosed in Pat. No. 5,472,796 include copper alloy C6155 having a nominal composition, by weight, of 92% copper, 6% aluminum and 2% nickel; the Swedish Crown having a nominal composition, by weight, of 89% copper, 5% aluminum, 5% zinc and 1% tin and the UK Pound having a nominal composition, by weight, of 70% copper, 24.5% zinc and 5.5% nickel.
U.S. Pat. Nos. 4,401,488; 4,330,599 and 5,472,796 are incorporated by reference in their entireties herein.
A second requirement of a new dollar coin is that the electrical signature be substantially identical to that of the SBA. Automatic coin discriminators, such as found in vending machines, utilize discriminators to determine the authenticity and face value of a coin. One type of automatic coin discriminator, referred to as an eddy current gauge, positions the coin adjacent to an electrical coil in which an alternating current is flowing. The alternating current, called the exciting current, causes eddy currents to flow in the coin. The magnitude and timing of the eddy currents is a function of the electrical conductivity of the coin. The SBA has an electrical conductivity in the transverse direction (from one major planar face to an opposing major planar face) of about 49% IACS when the exciting current is at a frequency of 60 kHz and about 6.6% IACS when the exciting current is at a frequency of 480 kHz.
IACS refers to xe2x80x9cInternational Annealed Copper Standardxe2x80x9d and assigns a conductivity value of 100% IACS to xe2x80x9cpurexe2x80x9d copper at 20xc2x0 C.
U.S. Pat. No. 4,525,434 discloses copper alloys containing manganese, zinc, nickel and aluminum. Optional additions to the alloy include iron, cobalt and tin. The claimed alloy is disclosed as having good oxidation resistance and utility in clad leadframes. The highest recited electrical conductivity for a claimed alloy is 3.1% IACS. U.S. Pat. No. 4,525,434 is incorporated by reference in its entirety herein.
A third requirement of a new one dollar coin is tarnish resistance to enable the coin to maintain a golden appearance for an extended period of time, on the order of years.
It is not believed that any of the gold-colored clad materials presently available satisfy all three of the above requirements for coinage: golden visual appearance, an electrical signature similar to that of the SBA and tarnish resistance. Accordingly, there remains a need for such a material for coinage.
Accordingly, it is an object of the invention to provide a copper-base alloy having a golden visual appearance and useful for coinage. Further objects of the invention are that the copper-base alloy has an electrical signature similar to that of copper alloy C713 and suitable conductivity in excess of 4% IACS at eddy current gauge exciting frequencies of between 60kHz and 480kHz.
The above-stated objects, features and advantages will become more apparent from the specification and drawings that follow.
It is a feature of the invention that the copper-base alloy may effectively be utilized as a monolithic material or as a cladding layer. When utilized as a cladding layer, the core is formed from either copper or a copper base alloy having high electrical conductivity, typically in excess of 90% IACS. In a preferred embodiment of the invention, the core is formed from copper alloy C110.
Yet another feature of the invention is that the copper-base alloy has a golden visual appearance and an electrical conductivity on the order of 5%-7% IACS. Yet another feature of the invention is that the copper-base alloy contains manganese and zinc. In a preferred embodiment, between 2% and 6%, by weight, of nickel is added to the copper-base alloy to improve tarnish resistance.
It is another feature of the invention that when a clad is formed, the core constitutes about 50% of the overall thickness of the clad material and that each cladding layer constitutes about 25% of the overall thickness.
Among the advantages of the invention are that the copper-base alloy has a golden visual appearance and is suitable as a planchet for the minting of coins in either a monolithic format or as a cladding layer. It is another advantage of the invention that the electrical signature of the clad material is similar to that of the SBA when measured with an eddy current gauge at frequencies between at least 60 kHz and 480 kHz. This advantage enables continued use of electronic coin discriminators now in service to identify the SBA.
In accordance with the invention, there is provided a copper alloy that consists essentially, by weight, of from 5% to 10% of manganese, from 10% to 14% of zinc, from 2% to 6% of nickel, and the balance copper and inevitable impurities. The copper alloy has an electrical